1. Field of the Invention
The present invention relates to a hydraulic power steering unit that assists steering.
2. Description of the Related Art
Japanese Patent Application Publication JP-A-2001-163233 describes a hydraulic power steering unit in which an oil pump includes a discharge port that has a control valve. The oil pump supplies a flow of oil for assisting steering from the control valve to a power cylinder in accordance with various signals for the steering angle, steering speed and vehicle speed during steering, whereby the hydraulic power steering unit generates assist force for assisting steering.
According to the above unit, oil is supplied to an assist force generating unit to allow the assist force to be generated in conjunction with steering, thereby assisting the driver's steering. However, a flow rate of oil discharged from the oil pump per unit time tends to change in response to changes in speed (rotational speed) of the oil pump. Therefore, if the oil pump speed changes, even if the steering conditions, i.e. vehicle speed and steering angle, are the same, the assist force may subtly differ before and after the change of the pump speed. This may cause a subtle difference in steering feel which the driver or other vehicle users can sense. In particular, when a transmission shifts gear, the pump speed easily changes due to change in the engine speed before and after the shifting. As a result, the flow rate of oil discharged from the oil pump per unit time tends to change. Accordingly, assist force may subtly differ before and after shifting, causing a difference in steering feel which the driver or other vehicle users can sense.